It has been shown that the addition of plant sterols, such as. .beta.-sitosterol, to diets will reduce serum cholesterol levels. The sterols reduce serum cholesterol through the disruption of intestinal absorption of dietary cholesterol by displacing it from bile acid micelli. More recently, .beta.-sitosterol's saturated derivative, .beta.-sitostanol, has been shown to be more effective in the reduction of intestinal cholesterol absorption. The sitostanol itself is virtually unabsorbed, so it does not contribute at all to in vivo serum sterol concentration upon consumption. Unfortunately, typical sterols and stanols are insoluble in the micelli phase of the alimentary canal and have only limited solubility in oils and/or fats or water. Hence, free sterols or stanols themselves are not optimum candidates for use in typical pharmaceutical or dietary dosage forms as cholesterol reducing agents.
U.S. Pat. No. 5,502,045 discloses the interesterification of stanols with a fatty acid ester from an edible oil to produce a waxy sterol--ester mixture with improved fat solubility characteristics. Specifically, this patent discloses the reaction of sitostanol interesterified to an edible oil such as rapeseed oil specifically via a base catalyzed transesterification reaction. This is a process that is widely used in the food industry. From a pharmaceutical standpoint, however, interesterification processes such as this have some distinct disadvantages. Primarily, the composition profile of the sterol-ester products are difficult to control since the profile is dependent on the array of fatty acids present in the edible oil employed in the reaction.
In a different approach, German Patent 2035069 discloses the esterification of sterol-esters to fatty acids via a non-food grade process. In particular, thionyl chloride is employed as a reactant which when reacted forms HCl gases as a by-product. Such techniques are not suitable for the production of food grade materials, and they are undesirable in general for large scale reactions.
From a pharmaceutical standpoint, there is an unmet need for a method for the synthesis of discreet stanol/sterol-esters via a bulk food grade process. Discrete compounds are more desirable than mixtures for three main reasons: 1) the composition and performance specifications can be controlled better; 2) structure/activity studies are more feasible; and 3) the physicochemical and chemical properties can be controlled. These advantages of discrete stanol/sterol-esters will be elaborated on later.